Question

A 1 cm object is placed 20 cm from a lens. The lens forms a real and inverted image of size 1.5 cm. What is the focal length of the lens?

Answer 1

Answer:

magnification, m = v/u or image height/object height  (v=image distance, u=object distance)

image height, h^i = 1.5

object height, h^o = 1

∴ m = 1.5/1

       = 1.5

object distance, u = 20cm

⇒ m = v/u

    1.5 = v/ 20

    ∴v = 30cm

now, using lens formula,

1/v + 1/u = 1/f

1/30 + 1/20 = 1/f

30+20/600 = 1/f

50/600 = 1/f

5/60 = 1/f

hence, 1/12 = 1/f (taking reciprocal on both sides we get f/1 = 12/1)

∴ focal length, f = 12 cm

Explanation: